1. Field of the Invention
The present invention relates to a transfer molding apparatus and a method of manufacturing semiconductor devices.
2. Related Art
In the manufacture of semiconductor devices, transfer molding apparatuses for encapsulating semiconductor devices mounted on lead frames are conventionally used. As shown in FIG. 7, the transfer molding apparatus comprises a transfer pot 10 into which solid thermosetting resin (tablets) is loaded, a plunger 12 for transferring the thermosetting resin 28 (hereafter referred to as the resin 28. See FIG. 8) that has been fluidified in the transfer pot, a position sensor 14 for detecting the position of the plunger 12, a top-half mold 16a fixed to a top platen 26a, a bottom-half mold 16b fixed to a bottom-half platen 26b, heaters 18 for heating the top-half and bottom-half molds 16a, 16b to a predetermined temperature, and a suction pump 24 for reducing the pressure in the cavities 20 by extracting the air from a chamber 30 where the top-half and bottom-half molds 16a, 16b are disposed.
When the top-half and bottom-half molds 16a, 16b are closed, two cavities 20 each for setting up a molding space for a plastic IC package, and runners 22 and gates 23 as resin supply paths leading to the cavities 20 are formed.
When the top-half and bottom-half molds 16a, 16b are closed, air-vent slots 25 are also formed at the outer end positions of the two cavities 20 opposite the gates 23 located at the inner ends thereof. When the chamber 30 is placed at reduced pressure by extracting air by a suction pump 24, the air is sucked out from the runners 22 and the gates 23 through the air-vent slots 25, so that the cavities 20 are placed at reduced pressure. The runners 22 guide the resin 28 into the cavities 20 through the gates 23 that are open to the corresponding cavities 20.
Referring to FIG. 8, description will be made of a method of manufacturing semiconductor devices on a transfer molding apparatus structured as described above. FIG. 8 shows only the principal portions for convenience of explanation. First of all, a semiconductor-device-mounted lead frame (not shown) is set in the bottom-half mold 16b, a resin tablet is loaded in the transfer pot 10, and by lowering the top platen 26a, the top-half mold 16a and the bottom-half mold 16b are closed, so that a cavity 20, for example, is formed as shown in FIG. 8(a). At this point in time, the semiconductor device has been placed almost in the center of the cavity 20. In addition, the pressure in the chamber 30 has been reduced to about 30 to 99 Pa by the suction pump 24.
While the resin tablet charged in the transfer pot 10 is being melted by heating it to 160.degree. to 190.degree. C. with the heaters 18, the resin is extruded from the transfer pot 10 by raising the plunger 12. By this operation, the molten resin 28 is introduced into the runner 22 as shown in FIG. 8(b).
By the increasing the forcing pressure from the plunger 12, the resin 28 in the runner 22 is guided through the gate 23 into the cavity 20 as shown in FIG. 8(c). As shown in FIG. 8 (d), when the resin 28 has been filled into the cavity 20, the forcing pressure from the plunger 12 is stopped, and the resin 28 in the cavity 20, the runner 22, and the gate 23 is cured. After the resin 28 is cured sufficiently, the top platen 26a (see FIG. 7) is raised, and the semiconductor device with a lead frame in a package of resin 20 that hardened around the semiconductor element is ejected. Subsequently, the excess resin is removed and whittled down to shape, and the lead-frame portion is cut off and the outer leads are formed to thereby produce a semiconductor package. Subsequently, the excess resin is removed, the package is whittled down to shape, the frame portion of the lead frame is trimmed, and the outer leads are formed. Thus, a semiconductor device is produced.
In the transfer molding apparatus constructed as described, there are possibilities of an unfilled region (voids) 29 being formed in the top-cavity portion or the bottom-cavity portion of the mold due to a difference in resin-filling speed between the top-cavity portion and the bottom-cavity portion, which partition is made by the semiconductor element loaded in the cavity 20. Voids are unwanted because they give rise to a warp or deformation in the package or decreases its strength or humidity resistance.
There have been countermeasures against the voids. One is to provide a suction port communicating with the cavity, and directly reduce the pressure in the cavity by the use of a suction pump to decrease the remaining air in the top-cavity portion or the bottom-cavity portion to prevent the occurrence of voids.
The other is to place the chamber 30 itself in a reduced-pressure atmosphere so that the remaining air in the top-cavity portion or the bottom-cavity portion should be extracted through the air-vent slot 25 and to thereby prevent the occurrence of voids.
However, in the transfer molding apparatus constructed as described above, because the resin passes through the gate of a smaller diameter than that of the runner when it enters a cavity, the resin is subjected to pressure at the gate, and the resin in compressed state is injected into the cavity. Therefore, if the cavity is at reduced pressure when a specified amount of resin is introduced into the cavity, there is a relatively large pressure difference between the pressure in the cavity and the pressure in the resin. A problem here is that when there is such a large pressure difference, the air bubbles in the resin expand notably, and remain as voids in the package.
Thermosetting resins have a characteristic that curing does not progress in proportion to the passage of time, but curing occurs after the viscosity decreases once. Therefore, with some kinds of thermosetting resins, the viscosity sometimes drops temporarily while the cavity is being filled with a molding compound. Also in this case, there is a problem that the air bubbles expand remarkably in the resin and remain as voids in the package.